1. Field of the Invention
The present invention relates to a method of correcting halftone dot image data in order to prevent tone jump from occurring in a halftone dot image which is generated under desired output conditions from the halftone dot image data that are converted from continuous tone image data, and an image processing apparatus capable of correcting such halftone dot image data.
2. Description of the Related Art
In the field of printing and platemaking, there have been employed image reading and recording apparatus for electrically processing image information of a color original and making a film with a view to increasing the efficiency of processing steps and improving the quality of images. In such image reading and recording apparatus, more specifically, the image information recorded on the original is photoelectrically read and converted into image data, which are subjected to various forms of image processing such as sharpness enhancement, tone adjustment, color correction, and the like. Thereafter, a laser beam modulated by the processed image data is scanned over a film to make an original film.
The image on an original film thus produced is recorded on a printed material normally by reversing the negative film into a positive film, producing a press plate from the positive film, and printing the image with the press plate.
A continuous tone image such as a photographic image is usually outputted onto a recording medium such as an original film, a printed material, or the like as follows: Continuous tone image data obtained from a photographic image or the like are first converted into halftone dot image data, and then a halftone dot image is recorded on a recording medium based on the halftone dot image data.
The halftone dot image has its continuous tone expressed by pixels so as to achieve a halftone dot area percentage depending on the continuous tone of the image data. For example, as shown in FIG. 4 of the accompanying drawings, a halftone dot image 2 having a halftone dot area percentage of 50% is produced by blackening pixels 6 in regions 4a, 4b. The regions 4a, 4b are held in contact with each other at a point 8. In the image reading and recording apparatus, however, since a scanning laser beam spot applied to the recording medium has a certain spatial distribution, the laser beam spots that form those pixels 6 which are positioned adjacent to the point 8 overlap each other when the image 2 is formed. Consequently, the halftone dot area percentage of 50% cannot be obtained because the point 8 is widened. Furthermore, there may be instances where no accurate halftone dot area percentage can be achieved in various processes following the process of generating the original film with the image reading and recording apparatus.
FIGS. 5A through 5D of the accompanying drawings show the relationship between continuous tone image data and the halftone dot area percentage on an original film in the process of generating the original film, the relationship between the halftone dot area percentage on the original film and the halftone dot area percentage on a positive film in the process of reversing the negative film into the positive film, the relationship between the halftone dot area percentage on the positive film and the halftone dot area percentage on a press plate in the process of producing the press plate, and the relationship between the halftone dot area percentage on the press plate and the halftone dot area percentage on a printed material in the process of printing a halftone dot image on the printed material, respectively. FIG. 5E of the accompanying drawings illustrates the relationship between the continuous tone image data and the halftone dot area percentage on the printed material based on the accumulation of the relationships shown in FIGS. 5A through 5D.
Therefore, the halftone dot area percentage of output data in each of the above processes and the desired halftone dot area percentage of input data tend to differ from each other depending on output conditions as to whether image data are to be outputted using a laser beam, whether a negative film is to be reversed into a positive film, whether a press plate is to be produced, whether image data are to be printed using ink, and how a screen ruling, a screen angle, and a halftone dot shape are to be set. If the output image data vary greatly from the input image data, then the continuous tone is apt to become discontinuous before and after an area where the output image data differ from the input image data. In the case where a number of processes are involved from the generation of an original film to the printing of a halftone dot image, variations or fluctuations caused in the respective processes are accumulated as shown in FIG. 5E, resulting in an excessive tone jump in the printed halftone dot image.
It is therefore an object of the present invention to provide a method of correcting halftone dot image data in order to produce a high-quality halftone dot image free of tone jump from the halftone dot image data under desired output conditions which have been converted from continuous tone image data, and an image processing apparatus capable of correcting such halftone dot image data.
The above object can be achieved by an image processing apparatus for converting continuous tone image data into halftone dot image data and generating a halftone dot image from the halftone dot image data, comprising continuous tone correction data memory means for storing continuous tone correction data corresponding to given output conditions, continuous tone image data correcting means for correcting continuous tone image data based on the continuous tone correction data, and converting means for converting the corrected continuous tone image data into halftone dot image data.
The image processing apparatus may also have threshold data memory means for storing threshold data corrected depending on given output conditions, and converting means for converting continuous tone image data into halftone dot image data based on the threshold data.
The given output conditions may include at least one of a type of a recording medium on which the halftone dot image is to be outputted, an exposure condition for the halftone dot image, a printing condition for the halftone dot image, a halftone dot shape for the halftone dot image, a screen ruling for the halftone dot image, and a screen angle for the halftone dot image.
According to the present invention, the above object can also be accomplished by a method of correcting halftone dot image data in converting continuous tone image data into halftone dot image data and generating a halftone dot image from the halftone dot image data under given output conditions, comprising the steps of correcting continuous tone image data based on continuous tone correction data produced for generating a halftone dot image having a given halftone dot area percentage under given output conditions, converting the corrected continuous tone image data into halftone dot image data, and generating a halftone dot image based on the halftone dot image data.
The above object can further be achieved by a method of correcting halftone dot image data in converting continuous tone image data into halftone dot image data and generating a halftone dot image from the halftone dot image data under given output conditions, comprising the steps of converting continuous tone image data into halftone dot image data based on threshold data produced for generating a halftone dot image having a given halftone dot area percentage under given output conditions, and generating a halftone dot image based on the halftone dot image data.
With the above arrangement, continuous tone correction data produced for allowing a halftone dot image produced under desired output conditions to have a desired halftone dot area percentage are stored in the continuous tone correction data memory means. After supplied continuous tone image data have been corrected by the continuous tone image data correcting means based on the continuous tone correction data, the corrected continuous tone image data are converted into halftone dot image data, which are then outputted as a halftone dot image. If the continuous tone correction data are established under various output conditions including the type of a recording medium on which the halftone dot image is to be outputted, the exposure condition for the halftone dot image, the printing condition for the halftone dot image, the halftone dot shape for the halftone dot image, the screen ruling for the halftone dot image, and the screen angle for the halftone dot image, then it is possible to produce a high-quality halftone dot image free of tone jump.
Furthermore, threshold data produced for allowing a halftone dot image produced under desired output conditions to have a desired halftone dot area percentage are stored in the continuous tone correction data memory means, and supplied continuous tone image data are converted into halftone dot image data by the converting means based on the threshold data, after which the halftone dot image data are outputted as a halftone dot image.
Furthermore, a halftone test image is produced from test data under desired output conditions, and the halftone dot area percentage of the produced halftone test image is measured. Then, correction data are determined for correcting the measured halftone dot area percentage into a halftone dot area percentage corresponding to the test data, and halftone dot image data are generated from continuous tone image data based on the correction data. Inasmuch as the correction data are established depending on the desired output conditions, it is possible to produce a high-quality halftone dot image free of tone jump from the halftone dot image data.